1. Field of the invention
The present invention relates to spark plugs for internal combustion engines and, more particularly, to a spark plug for use in a motor vehicle, a cogeneration system and a gas pressure feed pump or the like.
2. Description of the Related Art
In the related art, attempts have heretofore been made to provide spark plugs for use in internal combustion engines to be used as igniting means of the internal combustion engines of motor vehicles or the like.
The spark plugs usually include center electrodes and ground electrodes between which spark discharge gaps are provided. Applying a high voltage across the center electrode and the ground electrode allows a spark discharge to take place in the spark discharge gap, thereby igniting an air-fuel mixture.
In recent years, with an increasing demand for a low fuel consumption and high power output ratio needed for the engine of the motor vehicle, modern motor vehicles generally employ direct fuel-injection type engines each arranged to directly inject fuel into a combustion chamber of the engine. With such an arrangement, there has been an increasing trend in which an air fuel mixture, supplied to a combustion chamber, has an increased concentration of fuel in areas near the center electrode and the ground electrode around the spark discharge gap.
Therefore, this causes fuel of the air-fuel mixture to adhere onto the center electrode and the ground electrode especially at areas around the spark discharge gap. In particular, a tendency has occurred for fuel to adhere onto a surface of the ground electrode due to its large exposed area. This results in a phenomenon wherein fuel, adhered onto the surface of the ground electrode, flows along the surface of the ground electrode to a facing surface placed in face-to-face relationship with a distal end of the center electrode and clumps on the ground electrode. This causes a risk to occur for fuel of the air-fuel mixture to clump at the facing surface of the ground electrode (in a manner referred to as fuel clamping) at an increased fuel clamping rate. Another risk takes place for fuel of the air-fuel mixture clumped at the facing surface of the ground electrode causing a bridging to take place in the spark discharge gap to make connection between the distal end of the center electrode and the facing surface of the ground electrode (in a manner referred to as fuel bridging).
Such risks become serious especially when starting up the engine in an extremely low temperature environment, under which fuel clamping and fuel bridging are liable to occur at increased incidence rates.
With a view to addressing such an issue, an attempt has heretofore been made to provide a spark plug formed in a structure including a center electrode and a ground electrode both of which carries thereon noble metal chips formed in respective narrowed outer diameters for thereby suppressing the occurrence of fuel bridging (see Japanese Unexamined patent Application Publication No. 2001-307858).
However, even with such a spark plug being employed, an issue still arises especially in the direct fuel-injection type engine of the type discussed above in that the spark plug suffers the occurrence of fuel clamping and fuel bridging. Further, the provision of the spark plug with both of the center electrode and the ground electrode carrying thereon the noble metal chips results in an increase in man-hours and production.
In addition, it is conceived that fuel clamping and fuel bridging occur on the spark plug due to shapes of or positional relationship between the center electrode and the ground electrode. Thus, it can be considered that there still exists a room left for improvement of the spark plug in respect of the ground electrode.